X. Wang et al., Evidence that intragenic recombination contributes to allelic diversity ofthe S-RNase gene at the self-incompatibility (S) locus in Petunia inflata, PLANT PHYSL, 125(2), 2001, pp. 1012-1022
For Solanaceae type self-incompatibility, discrimination between self and n
onself pollen by the pistil is controlled by the highly polymorphic S-RNase
gene. To date, the mechanism generating the allelic diversity of this gene
is largely unknown. Natural populations offer a good opportunity to addres
s this question because they likely contain different alleles that share re
cent common progenitors. We identified 19 S haplotypes from a natural popul
ation of Petunin inflata in Argentina, used reverse transcriptase-polymeras
e chain reaction to obtain cDNAs for 15 alleles of the S-RNase gene, and se
quenced all the cDNAs. Phylogenetic studies revealed that five of these all
eles and two previously identified alleles form a major clade, and that the
5' region of S-19 allele was derived from an ancestor allele closely relat
ed to S-2, whereas its 3' region was derived from an ancestor allele closel
y related to S-8. A similar evolutionary relationship was found among S-3,
S-12, and S-15 alleles. These findings suggest that intragenic recombinatio
n contributed to the generation of the allelic diversity of the S-RNase gen
e. Two additional findings emerged from the sequence comparisons. First, th
e nucleotide sequence of the S-1 allele identified in this work is complete
ly identical to that of the previously identified S-1 allele of a different
origin. Second, in the two hypervariable regions HVa and HVb, thought to b
e involved in determining S allele specificity, S-6 and S-9 alleles differ
only by four nucleotides, all in HVb, resulting in two amino acid differenc
es. The implications of these findings are discussed.